Intermittent heavy precipitation was observed across the South Coast of British Columbia (BC), Canada, on 11 January 2010, as an atmospheric river (AR) – a narrow band of enhanced water vapour flux in the troposphere (a.k.a Pineapple Express) – moved onshore. Numerous transverse cloud bands, manifesting as gravity waves embedded in and travelling along the AR, were clearly identified in satellite imagery. These mesoscale (15–30 km) gravity waves enhanced significantly within sub-synoptic scale (300–600 km) envelopes downstream of the AR. Radar imagery showed that mesoscale rain bands developed over the Olympic Mountains and the southern Vancouver Island mountain ranges as a larger gravity wave envelope moved onshore. Detailed analysis of intensive observations suggested that the wave-induced rain bands and the associated seeder-feeder/spillover effects made a large contribution to the heavy precipitation on both the windward and lee sides of the coastal mountains in southern BC.

There was compelling evidence to suggest that both thermal and dynamical instabilities were involved with the development of the gravity waves. Both of the boundary-layer Kelvin-Helmholtz (KH) instability and the conditional instability were identified in the upstream portion of the AR. These unstable conditions excited upward propagating gravity waves. A mid-tropospheric layer with weak KH instability appeared to support a wave-CISK mode and give rise to wave over-reflection/over-transmission, which could be considered as the primary source of the enhanced wave packets observed in the AR downstream. As the AR moved onshore toward a complex mountain terrain, resonance could result from constructive interference of the incoming travelling waves and the orographically-forced stationary waves, leading to further wave enhancement and possible wave breaking in multiple layers with KH instability in the upper troposphere. Conceptual models were developed to aid operational forecasting/nowcasting of this event.